This invention relates to Dense Wavelength Division Multiplexer (DWDM) devices, and more particularly to DWDM cascade structures that incorporate Mach-Zehnder interferometers.
A DWDM device can be used to increase the number of communication channels available in a fiber optical system. Today, researchers are studying a few competing technologies. One mature technology, relying on standard thin film filter coatings, is characterized by high signal insertion losses, low channel counts and relatively high cost, and is generally useful only for DWDM devices with channel spacing greater than 50 GHz.
Another competing technology, involving use of Mach-Zehnder interferometers (xe2x80x9cMZIsxe2x80x9d), is characterized by low signal insertion loss, low polarization dependent low, relatively low cost, high uniformity and high signal crosstalk and is a more attractive choice for DWDM devices with lower channel spacing. However, standard MZI technology suffers from low isolation between adjacent channels and provides an approximately Gaussian shape for the corresponding transmission curves. These latter two problems make it difficult for a DWDM device relying on standard MZI technology to comply with DWDM standards for optical isolation and xe2x80x9cflat-topxe2x80x9d passband response set down by BellCore. If these two problems can be either solved or reduced in severity, DWDM devices relying on MZI technology could become widely used in voice, data and image communications.
What is needed is a DWDM system having low signal insertion loss, low polarization dependent loss, high uniformity, relatively low signal crosstalk, acceptable channel isolation and acceptably low passband insertion loss. Preferably, the system should have acceptably low cost and should be flexible enough to meet various commercial communication requirements. Preferably, the system should meet or exceed the BellCore standards for optical isolation and for xe2x80x9cflat-topxe2x80x9d passband response.
These needs are met by the invention, which uses an improved DWDM cascade structure with distributed filtering and MZI technology to provide acceptable channel isolation for relatively low channel spacing and to comply with the BellCore standards for optical isolation and for flat-top passband response within a channel, over a system of 2N output channels for 25 or 50 GHz (or higher) channel spacing with N=4, 5, 6, . . .
The basic structure is a bifurcated or cascade tree system with N stages, numbered n=1, 2, . . . N, with stage number n having 2n fiber optical channels in parallel, with each channel in stage n having an MZI, defined by two 3 dB couplers and two parallel fiber optic arms of unequal length, at the beginning of the channel, and with each channel except an output channel or port feeding an MZI that is part of stage n+1(n=1, 2, . . . , Nxe2x88x921). A xe2x80x9cstagexe2x80x9d, as used herein, refers to a group of one or more parallel fiber optic channels, with each channel having an MZI positioned at the beginning of the channel for wavelength discrimination. A typical cascade tree structure of fiber optic channels is disclosed and discussed in U.S. Pat. Nos. 5,809,190 and 5,987,201, issued to P. Z. Chen (FIG. 1 and discussion), incorporated by reference herein.
The channels in three independently selected stages, preferably an input channel (xe2x80x9cstage 0xe2x80x9d) and the second and third stages, include one (or more) of several types of special purpose filters. A first filter type is an xe2x80x9cinvertedxe2x80x9d Fabry-Perot etalon (xe2x80x9cFPExe2x80x9d), in which wavelengths for one or more transmission minima coincide with selected wavelengths. Each inverted FPE has a relatively small optical finesse F, preferably Fxe2x89xa62, and has a small selected free spectral range (xe2x80x9cFSRxe2x80x9d) corresponding to the difference between two consecutive wavelengths at which the transmission minima occur.
A second filter type is an xe2x80x9cinvertedxe2x80x9d and non-symmetric MZI, in which wavelengths for one or more transmission minima coincide with selected wavelengths. A non-symmetric MZI does not use first and second 3 dB (50 percent) couplers, one at each end, to couple the signals in each arm of the MZI, but uses two partial couplers, with at least one of the two coupling coefficients differing from 0.5 and being selected to satisfy selected criteria. A filter of the first type or of the second type is incorporated in each channel in a selected stage of the system. Inclusion of a filter of the first type or of the second type provides a flattening of a transmission peak in that channel at each of the selected wavelengths, to facilitate compliance with the BellCore standards for flat-top response.
A third type of filter is a standard (symmetric) MZI, defined by two 3 dB couplers and serving as a filter with a specified FSR, to deepen the optical isolation at specified wavelengths and to facilitate compliance with the corresponding BellCore standard. A fourth type of filter is a conventional FPE, having a modest optical finesse F (1xe2x89xa6F less than 6) and serving as a filter with a specified FSR, to deepen the optical isolation at specified wavelengths and to facilitate compliance with the corresponding BellCore standard. The third type of filter and, separately, the fourth type of filter is incorporated in each channel in one of the selected stages 0-3 in the system.
In one embodiment, involving 50 GHz channel spacing with N=5 stages and 32 output channels, an optimum choice of parameters is F(type 1)=0.7, FSR(type 1)=0.8/4=0.2 nm, xcex1(type 2)=0.04, FSR(type 2)=0.8/4=0.2 nm, FSR(type 3)=1.6 nm, F(type 4)=6 and FSR(type 4)=3.2 nm.